1. Field of the Invention
The present invention relates to a dielectric resonator including a dielectric core and a cavity. The present invention also relates to a filter and a duplexer using such a dielectric resonator and to a communication device including such a filter or a duplexer.
2. Description of the Related Art
Conventionally, a small-sized dielectric resonator including a dielectric core disposed in a cavity is capable of handling relatively high power in a microwave range.
For example, a dielectric resonator using a TM mode is formed by disposing a dielectric core of dielectric ceramic in a cavity of a cavity member formed of metal or ceramic the surface of which is covered with an electrode film.
An example of a structure of a conventional dielectric resonator is shown in FIGS. 18, 19A, and 19B, wherein FIG. 18 is an exploded perspective view, FIG. 19A is a top view, and FIG. 19B is a cross-sectional view. In this example, the dielectric resonator is formed as follows. A dielectric core 3 having electrodes formed on two respective end faces thereof is inserted into a main portion 1 of a cavity member made of metal, and the two end faces of the dielectric core 3 are connected to the inner surface of the main portion 1 of the cavity member via solder 6 (see FIG. 19A-19B). Thereafter, the opening of the main portion 1 of the cavity member is closed with a cavity lid 2.
In the above structure in which both end faces of the dielectric core are bonded to the inner surface of the cavity member, if there is a large difference between the coefficient of linear expansion of the dielectric core and that of the cavity member, degradation occurs in the bonding portion between the dielectric core and the cavity member due to heat cycle fatigue, and thus sufficiently high reliability cannot be obtained.
One known technique to avoid the above problem is to form a dielectric core and a cavity member by means of a monolithic molding process. In this structure, because both the dielectric core and the cavity member are formed of the same ceramic material, there is essentially no problem due to the heat cycle fatigue.
However, this structure, formed by monolithically molding the dielectric core and the cavity member, is formed of dielectric ceramic, despite the fact that most of the cavity member does not need to be dielectric. Thus, the material cost increases. Besides, a complicated mold is needed and thus the production cost also increases.
Japanese Patent Application No. 11-283037 filed by the present applicant discloses a resonator formed by disposing a conducting bar together with a dielectric core into a cavity so that both a resonance mode associated with the dielectric core and a coaxial (semicoaxial) resonance mode are used. However, in this structure, there is a large difference between the linear expansion coefficient of the cavity member made of an ordinary metal material such as aluminum and that of the dielectric core, and thus sufficiently high reliability in the bonding portion between the dielectric core and the cavity member is not achieved for the above-described reason. The above problem can be solved if a metal material having a linear expansion coefficient similar to that of the dielectric ceramic material forming the dielectric core is employed to form the cavity member. However, the result is increased material cost for the cavity member and increased production cost needed to produce the cavity member.
Thus, there is a need for a dielectric resonator which has high durability against heat cycle fatigue in a bonding portion between an electrically conductive cavity member and a dielectric core disposed in the cavity member, and which can be produced without increasing the material cost and the production cost. There is also a need for a filter and a duplexer using such a dielectric resonator. There is further a need for a communication device including such a filter or a duplexer.
According to an aspect of the present invention, there is provided a dielectric resonator comprising: a dielectric core having an electrode formed on an end face thereof; an electrically conductive cavity member; and an electrically conductive foil having a bonding surface bonded to the end face and also having a bent spring portion, the bonding surface of the foil being adhesively bonded to the end face of the dielectric core via an electrically conductive adhesive, the spring portion of the foil being adhesively bonded to the inner surface of the cavity member via an electrically conductive adhesive.
In this dielectric resonator according to the present invention, the dielectric core preferably includes a flange portion formed on an end thereof, and the electrically conductive foil preferably includes a cover portion for covering an end face of the flange portion, and the spring portion of the electrically conductive foil is preferably formed by bending the cover portion along the edge of the flange portion.
According to another aspect of the present invention, there is provided a dielectric resonator comprising a dielectric core having an electrode formed on a particular end face thereof; an electrically conductive cavity member; and an electrically conductive foil, a central portion of which is raised to one side, the raised portion of the foil being adhesively bonded to the end face of the dielectric core via an electrically conductive adhesive, the spring portion of the foil being adhesively bonded to the inner surface of the cavity member via an electrically conductive adhesive.
In these structures described above, the end face of the dielectric core is elastically connected to the inner surface of the cavity member via the electrically conductive foil instead of being directly connected. As a result, distortion due to the difference between the linear expansion coefficient of the dielectric core and that of the cavity member is absorbed by the foil having elasticity, and thus no heat cycle fatigue occurs in the bonding portion between the dielectric core and the cavity member.
In this dielectric resonator according to the present invention, an adhesive is preferably inserted into the space surrounded by the raised portion so that electrical connection between the end face of the dielectric core and the cavity member is achieved via the electrically conductive foil, and mechanical connection between them is achieved via the foil and the adhesive. Because the end face electrode of the dielectric core and the cavity member are electrically connected to each other via the electrically conductive foil, no electric field enters the adhesive, and thus no degradation occurs.
In this dielectric resonator according to the present invention, preferably, the cavity member has a hole leading to the space surrounded by the raised portion, and the hole and the space surrounded by the raised portion are filled with an adhesive. This makes it possible to easily inject the adhesive from the outside of the cavity member. Furthermore, the cured adhesive is fitted in the hole and thus the bonding strength between the cavity member and the foil and the dielectric core is enhanced.
In this dielectric resonator according to the present invention, preferably, the dielectric core has a recessed and protruded portion formed on an end face thereof. This results in an increase in the bonding strength between the end face of the dielectric core and the adhesive in a shearing direction.
According to still another aspect of the present invention, there is provided a filter including a dielectric resonator having one of the structures described above; and a coupling structure which is coupled with an electromagnetic field in the resonance mode of the dielectric resonator and which serves as an signal input/output part.
According to still another aspect of the present invention, there is provided a duplexer including a filter formed of a plurality of dielectric resonators having one of the structures described above; and a coupling structure which is coupled with two of the plurality of dielectric resonators so that the coupling structure serves as a common antenna input/output terminal.
According to still another aspect of the present invention, there is provided a communication device including the filter or the duplexer described above.
Other features and advantages of the present invention will become apparent from the following description of embodiments of the invention which refers to the accompanying drawings, in which like references denote like elements and parts.